infra: Optimize Makefile for Better Usability and Maintenance (#18859)

**Previous screenshots：**


![image](https://github.com/langchain-ai/langchain/assets/86140903/e2f326e3-4d97-4b22-aacb-e789a9d815e4)

**Current screenshot：**

![image](https://github.com/langchain-ai/langchain/assets/86140903/bd8a3ea7-1b8a-4803-9168-df45f6fa4893)

Makefile

@@ -1,44 +1,56 @@
-.PHONY: all clean docs_build docs_clean docs_linkcheck api_docs_build api_docs_clean api_docs_linkcheck
+.PHONY: all clean help docs_build docs_clean docs_linkcheck api_docs_build api_docs_clean api_docs_linkcheck spell_check spell_fix lint lint_package lint_tests format format_diff
 
-# Default target executed when no arguments are given to make.
+## help: Show this help info.
+help: Makefile
+	@printf "\n\033[1mUsage: make <TARGETS> ...\033[0m\n\n\033[1mTargets:\033[0m\n\n"
+	@sed -n 's/^##//p' $< | awk -F':' '{printf "\033[36m%-30s\033[0m %s\n", $$1, $$2}' | sort | sed -e 's/^/ /'
+
+## all: Default target, shows help.
 all: help
 
+## clean: Clean documentation and API documentation artifacts.
+clean: docs_clean api_docs_clean
 
 ######################
 # DOCUMENTATION
 ######################
 
-clean: docs_clean api_docs_clean
+## docs_build: Build the documentation.
-
-
 docs_build:
 	docs/.local_build.sh
 
+## docs_clean: Clean the documentation build artifacts.
 docs_clean:
 	@if [ -d _dist ]; then \
-			rm -r _dist; \
+		rm -r _dist; \
-			echo "Directory _dist has been cleaned."; \
+		echo "Directory _dist has been cleaned."; \
 	else \
-			echo "Nothing to clean."; \
+		echo "Nothing to clean."; \
 	fi
 
+## docs_linkcheck: Run linkchecker on the documentation.
 docs_linkcheck:
 	poetry run linkchecker _dist/docs/ --ignore-url node_modules
 
+## api_docs_build: Build the API Reference documentation.
 api_docs_build:
 	poetry run python docs/api_reference/create_api_rst.py
 	cd docs/api_reference && poetry run make html
 
+## api_docs_clean: Clean the API Reference documentation build artifacts.
 api_docs_clean:
 	find ./docs/api_reference -name '*_api_reference.rst' -delete
 	cd docs/api_reference && poetry run make clean
 
+## api_docs_linkcheck: Run linkchecker on the API Reference documentation.
 api_docs_linkcheck:
 	poetry run linkchecker docs/api_reference/_build/html/index.html
 
+## spell_check: Run codespell on the project.
 spell_check:
 	poetry run codespell --toml pyproject.toml
 
+## spell_fix: Run codespell on the project and fix the errors.
 spell_fix:
 	poetry run codespell --toml pyproject.toml -w
 
@@ -46,31 +58,14 @@ spell_fix:
 # LINTING AND FORMATTING
 ######################
 
+## lint: Run linting on the project.
 lint lint_package lint_tests:
 	poetry run ruff docs templates cookbook
 	poetry run ruff format docs templates cookbook --diff
 	poetry run ruff --select I docs templates cookbook
 	git grep 'from langchain import' docs/docs templates cookbook | grep -vE 'from langchain import (hub)' && exit 1 || exit 0
 
+## format: Format the project files.
 format format_diff:
 	poetry run ruff format docs templates cookbook
 	poetry run ruff --select I --fix docs templates cookbook
-
-
-######################
-# HELP
-######################
-
-help:
-	@echo '===================='
-	@echo '-- DOCUMENTATION --'
-	@echo 'clean                        - run docs_clean and api_docs_clean'
-	@echo 'docs_build                   - build the documentation'
-	@echo 'docs_clean                   - clean the documentation build artifacts'
-	@echo 'docs_linkcheck               - run linkchecker on the documentation'
-	@echo 'api_docs_build               - build the API Reference documentation'
-	@echo 'api_docs_clean               - clean the API Reference documentation build artifacts'
-	@echo 'api_docs_linkcheck           - run linkchecker on the API Reference documentation'
-	@echo 'spell_check               	- run codespell on the project'
-	@echo 'spell_fix               		- run codespell on the project and fix the errors'
-	@echo '-- TEST and LINT tasks are within libs/*/ per-package --'